Introduction
Mammalian bone tissue is known to contain one or more proteinaceous materials, presumably active during growth and natural bone healing, that can induce a developmental cascade of cellular events resulting in endochondral bone formation. The active factors are variously been referred to in the literature as bone morphogenetic or morphogenic proteins (BMPs), bone inductive proteins, bone growth or growth factors, osteogenic proteins, or osteoinductive proteins. These active factors are collectively referred to herein as osteoinductive factors.
It is well known that bone contains these osteoinductive factors. These osteoinductive factors are present within the compound structure of cortical bone and are present at very low concentrations, e.g., 0.003%. Osteoinductive factors direct the differentiation of pluripotential mesenchymal cells into osteoprogenitor cells that form osteoblasts. Based upon the work of Marshall Urist as shown in U.S. Pat. No. 4,294,753, issued Oct. 13, 1981, proper demineralization of cortical bone exposes the osteoinductive factors, rendering it osteoinductive, as discussed more fully below.
Overview of Bone Grafts
The rapid and effective repair of bone defects caused by injury, disease, wounds, or surgery has long been a goal of orthopaedic surgery. Toward this end, a number of compositions and materials have been used or proposed for use in the repair of bone defects. The biological, physical, and mechanical properties of the compositions and materials are among the major factors influencing their suitability and performance in various orthopaedic applications.
Autologous cancellous bone (“ACB”) long has been considered the gold standard for bone grafts. ACB is osteoinductive and nonimmunogenic, and, by definition, it has all of the appropriate structural and functional characteristics appropriate for the particular recipient. Unfortunately, ACB is only available in a limited number of circumstances. Some individuals lack ACB of appropriate dimensions and quality for transplantation, and donor site pain and morbidity can pose serious problems for patients and their physicians.
Much effort has been invested in the identification and development of alternative bone graft materials. Urist has published seminal articles on the theory of bone induction and a method for decalcifying bone, i.e., making demineralized bone matrix (DBM). Urist M. R., Bone Formation by Autoinduction, Science 1965; 150(698):893-9; Urist M. R. et al., The Bone Induction Principle, Clin. Orthop. Rel. Res. 53:243-283, 1967. As mentioned above, DBM is an osteoinductive material, in that it induces bone growth when implanted in an ectopic site of a rodent, owing to the osteoinductive factors contained within the DBM. Honsawek et al. (2000). It is now known that there are numerous osteoinductive factors, e.g., BMP 1-15, which are part of the transforming growth factor-beta (TGF-beta) superfamily (Kawabata et al., 2000). BMP-2 has become the most important and widely studied of the BMP family of proteins. There are also other proteins present in DBM that are not osteoinductive alone but still contribute to bone growth, including fibroblast growth factor-2 (FGF-2), insulin-like growth factor-I and -II (IGF-I and IGF-II), platelet derived growth factor (PDGF), and transforming growth factor-beta 1 (TGF-beta.1) (Hauschka, et al. 1986; Canalis, et al, 1988; Mohan et al. 1996).
DBM implants have been reported to be particularly useful (see, for example, U.S. Pat. Nos. 4,394,370, 4,440,750, 4,485,097, 4,678,470, and 4,743,259; Mulliken et al., Calcif Tissue Int. 33:71, 1981; Neigel et al., Opthal. Plast. Reconstr. Surg. 12:108, 1996; Whiteman et al., J. Hand. Surg. 18B:487, 1993; Xiaobo et al., Clin. Orthop. 293:360, 1993, each of which is incorporated herein by reference). DBM typically is derived from cadavers. The bone is removed aseptically and treated to kill any infectious agents. The bone is particulated by milling or grinding, and then the mineral component is extracted by various methods, such as by soaking the bone in an acidic solution. The remaining matrix is malleable and can be further processed and/or formed and shaped for implantation into a particular site in the recipient. Demineralized bone prepared in this manner contains a variety of components including proteins, glycoproteins, growth factors, and proteoglycans. Following implantation, the presence of DBM induces cellular recruitment to the site of injury. The recruited cells may eventually differentiate into bone forming cells. Such recruitment of cells leads to an increase in the rate of wound healing and, therefore, to faster recovery for the patient.
Some studies indicate that the osteoinductive capabilities of demineralized bone from higher order species in higher order species is relatively low. One study compared the osteoinductivity of rat and canine bone matrix, and of cortical and cancellous bone. Rat bone matrix consistently induced new bone and high phosphatase levels when implanted ectopically in rat. Canine matrix induced small amounts of bone and lower phosphatase levels when implanted in dog and in rat, with cortical matrix being somewhat more inductive than cancellous matrix. Demineralized cancellous bone matrix from dog was the only material tested not showing any osteoinductivity. Schwarz et al., Acta. Orthop. Scan. 60(6):693-695, 1989.
Similarly, another study determined that monkey bone matrix induces ectopic bone formation in the athymic rat but not in adult monkeys. It was concluded that adult monkey bone matrix contains bone inductive properties but that these properties are not sufficient to induce bone formation in adult monkey muscle sites. Aspenberg et al., J. of Orthop. Res. 9:20-25, 1991.
Yet another study evaluated bone and cementum regeneration following guided tissue regeneration (GTR) in periodontal fenestration defects. Specifically, the adjunctive effect of allogenic, freeze-dried DBM implant was evaluated and found to exhibit no discernible adjunctive effect to GTR in the defect model. The critical findings were 1) the DBM particles remained embedded in dense connective tissue without evidence of bone metabolic activity; and 2) limited and similar amounts of bone and cementum regeneration were observed for both GTR plus DBM and GTR defects. Caplanis et al., J Periodontal 851-856, August, 1998.
Current DBM formulations have various drawbacks. First, while the collagen-based matrix of DBM is relatively stable, the osteoinductive factors within the DBM matrix are rapidly degraded. The osteogenic activity of the DBM may be significantly degraded within 24 hours after implantation, and in some instances the osteogenic activity may be inactivated within 6 hours. Therefore, the osteoinductive factors associated with the DBM are only available to recruit cells to the site of injury for a short time after transplantation. For much of the healing process, which may take weeks to months, the implanted material may provide little or no assistance in recruiting cells. In addition to the osteoinductive factors present within the DBM, the overall structure of the DBM implant is also believed to contribute to the bone healing capabilities of the implant.
Extracting Proteins
The potential utility of osteoinductive factors has been recognized widely. It has been contemplated that the availability of osteoinductive factors could revolutionize orthopedic medicine and certain types of plastic surgery, dental, and various periodontal and craniofacial reconstructive procedures.
Urist's U.S. Pat. No. 4,294,753, herein incorporated by reference, was the first of his many patents on a process for extracting BMP from DBM. At the time of the Urist '753 patent, BMP was referred to generally. However, as mentioned above, now it is known that there are multiple forms of BMP. The Urist process became widely adopted, and though different users may use different chemical agents from those disclosed in the basic Urist process, the basic layout of the steps of the process remains widely used today as one of the main methods of extracting BMP from DBM. See, e.g., U.S. Pub 2003/0065392 (2003); U.S. Pub 2002/0197297 (2002). Urist reported that his basic process actually results in generally low yields of BMP per unit weight of DBM. Urist et al. (1982).
The observed properties of osteoinductive factors have induced an intense research effort in several laboratories directed to isolating and identifying the pure factor or factors responsible for osteogenic activity. A modified process for purification of osteogenic protein from mammalian bone is disclosed by Sampath et al. (1987) Proc. Natl. Acad. Sci. USA 84:7109-7113. Urist et al. (1983), Proc. Soc. Exp. Biol. Med. 173:194-199, disclose a human osteogenic protein fraction which was extracted from demineralized cortical bone by means of a calcium chloride-urea inorganic-organic solvent mixture, and retrieved by differential precipitation in guanidine-hydrochloride and preparative gel electrophoresis. The authors report that the protein fraction has an amino acid composition of an acidic polypeptide and a molecular weight in a range of 17-18 kDa. This material was said to be distinct from a protein called “bone derived growth factor” disclosed by Canalis et al. (1980 Science 210:1021-1023) and by Farley et al. (1982) Biochem 21:3508-3513.
Urist et al., (1984) Proc. Natl. Acad. Sci. USA 81:371-375, disclose a bovine BMP extract having the properties of an acidic polypeptide and a molecular weight of approximately 18 kDa. The authors report that the protein was present in a fraction separated by hydroxyapatite chromatography, and that it induced bone formation in mouse hindquarter muscle and bone regeneration in trephine defects in rat and dog skulls. Their method of obtaining the extract from bone results in ill-defined and impure preparations.
European Patent Application Serial No. 148,155, published Oct. 7, 1985, herein incorporated by reference, purports to disclose osteogenic proteins derived from bovine, porcine, and human origin. One of the proteins, designated by the inventors as a P3 protein having a molecular weight of 22-24 kDa, is said to have been purified to an essentially homogeneous state. This material is reported to induce bone formation when implanted into animals.
International Application No. PCT/087/01537 (Int. Pub. No. WO88/00205) discloses an impure fraction from bovine bone with bone induction qualities. The named applicants also disclose putative “bone inductive factors” produced by recombinant DNA techniques. Four DNA sequences were retrieved from human or bovine genomic or cDNA libraries and expressed in recombinant host cells. While the applicants stated that the expressed proteins may be bone morphogenic proteins, bone induction was not demonstrated. This same group reported subsequently ((1988) Science 242:1528-1534) that three of the four factors induce cartilage formation, and postulate that bone formation activity “is due to a mixture of regulatory molecules” and that “bone formation is most likely controlled . . . by the interaction of these molecules.” Again, no bone induction was attributed to the products of expression of the cDNAs. See also Urist et al., EPO 0,212,474, entitled “Bone Morphogenic Agents.”
Wang et al., (1988) Proc. Nat. Acad. Sci. USA 85: 9484-9488, disclose the partial purification of a bovine bone morphogenetic protein from guanidine extracts of demineralized bone having cartilage and bone formation activity as a basic protein corresponding to a molecular weight of 30 kDa determined from gel elution. Separation of the 30 kDa fraction yielded proteins of 30, 18, and 16 kDa, which, upon separation, were inactive. In view of this result, the authors acknowledge that the exact identity of the active material had not been determined.
Wang et al., (1990) Proc. Nat. Acad. Sci. USA 87: 2220-2224, describe the expression and partial purification of one of the cDNA sequences described in PCT 87/01537. Consistent cartilage and/or bone formation with their protein requires a minimum of 600 ng of 50% pure material.
International Application No. PCT/89/04458 (Int. Pub. No. WO90/003733) describes the purification and analysis of a family of osteogenic factors called “P3 OF 31-34.”. The protein family contains at least four proteins, which are characterized by peptide fragment sequences. The impure mixture P3 OF 31-34 is assayed for osteogenic activity. The activity of the individual proteins is neither assessed nor discussed.
Implanting Extracted Proteins
Successful implantation of the osteoinductive factors for endochondral bone formation requires association of the proteins with a suitable carrier material capable of maintaining the proteins at an in vivo site of application. The carrier should be biocompatible, in vivo biodegradable, and porous enough to allow cell infiltration. Insoluble collagen particles that remain after guanidine extraction and delipidation of pulverized bone generally have been found effective in allogenic implants in some species. However, studies have shown that while osteoinductive proteins are useful cross species, the collagenous bone matrix generally used for inducing endochondral bone formation is species-specific. Sampath and Reddi, (1983) Proc. Nat. Acad. Sci. USA 80: 6591-6594. Demineralized, delipidated, extracted xenogenic bone matrix carriers implanted in vivo invariably fail to induce osteogenesis, presumably due to inhibitory or immunogenic components in the bone matrix. Even the use of allogenic bone matrix in osteogenic devices may not be sufficient for osteoinductive bone formation in many species, as discussed above.
U.S. Pat. No. 4,563,350, herein incorporated by reference, discloses the use of trypsinized bovine bone matrix as a xenogenic matrix to effect osteogenic activity when implanted with extracted, partially purified bone-inducing protein preparations. Bone formation is said to require the presence of at least 5%, and preferably at least 10%, non-fibrillar collagen. The named inventors claim that removal of telopeptides that are responsible in part for the immunogenicity of collagen preparations is more suitable for xenogenic implants.
European Patent Application Serial No. 309,241, published Mar. 29, 1989, herein incorporated by reference, discloses a device for inducing endochondral bone formation comprising an osteogenic protein preparation, and a matrix carrier comprising 60-98% of either mineral component or bone collagen powder and 2-40% atelopeptide hypoimmunogenic collagen.
Deatherage et al., (1987) Collagen Rel. Res. 7: 2225-2231, purport to disclose an apparently xenogenic implantable device comprising a bovine bone matrix extract that has been minimally purified by a one-step ion exchange column and reconstituted with highly purified human Type-I placental collagen.
U.S. Pat. No. 3,394,370, herein incorporated by reference, describes a matrix of reconstituted collagen purportedly useful in xenogenic implants. The collagen fibers are treated enzymatically to remove potentially immunogenic telopeptides (also the primary source of interfibril crosslinks), and are dissolved to remove associated noncollagenenous components. The matrix is formulated by dispersing the reconstituted collagen in acetic acid to form a disordered matrix of elementary collagen molecules that is then mixed with an osteogenic substance and lyophilized to form a “semi-rigid foam or sponge” that is preferably crosslinked. The formulated matrix is not tested in vivo.
U.S. Pat. No. 4,172,128, herein incorporated by reference, describes a method for degrading and regenerating bone-like material of reduced immunogenicity, said to be useful cross-species. Demineralized bone particles are treated with a swelling agent to dissolve any associated mucopolysaccharides (glycosaminoglycans), and the collagen fibers subsequently dissolved to form a homogenous colloidal solution. A gel of reconstituted fibers then can be formed using physiologically inert mucopolysaccharides and an electrolyte to aid in fibril formation.
The use of pulverized exogenous bone growth material, e.g., derived from demineralized allogenic or xenogenic bone, in the surgical repair or reconstruction of defective or diseased bone is known. See, in this regard, the disclosures of U.S. Pat. Nos. 4,394,370, 4,440,750, 4,472,840, 4,485,097, 4,678,470, and 4,743,259; Bolander et al., “The Use of Demineralized Bone Matrix in the Repair of Segmental Defects,” The Journal of Bone and Joint Surgery, Vol. 68-A, No. 8, pp. 1264-1273; Glowacki et al, “Demineralized Bone Implants,” Symposium on Horizons in Plastic Surgery, Vol. 12, No. 2; pp. 233-241 (1985); Gepstein et al., “Bridging Large Defects in Bone by Demineralized Bone Matrix in the Form of a Powder,” The Journal of Bone and Joint Surgery, Vol. 69-A, No. 7, pp. 984-991 (1987); Mellonig, “Decalcified Freeze-Dried Bone Allograft as an Implant Material In Human Periodontal Defects,” The International Journal of Periodontics and Restorative Dentistry, pp. 41-45 (June 1984); Kaban et al., “Treatment of Jaw Defects with Demineralized Bone Implants,” Journal of Oral and Maxillofacial Surgery, pp. 623-626 (Jun. 6, 1989); and Todescan et al., “A Small Animal Model for Investigating Endosseous Dental Implants: Effect of Graft Materials on Healing of Endosseous, Porous-Surfaced Implants Placed in a Fresh Extraction Socket,” The International Journal of Oral & Maxillofacial Implants Vol. 2, No. 4, pp. 217-223 (1987), all herein incorporated by reference.
A variety of approaches have been explored in an attempt to recruit progenitor cells or chondrocytes into an osteochondral or chondral defect. For example, penetration of subchondral bone has been performed in order to access mesenchymal stem cells (MSCs) in the bone marrow, which have the potential to differentiate into cartilage and bone. Steadman, et al., “Microfracture: Surgical Technique and Rehabilitation to Treat Chondral Defects,” Clin. Orthop., 391 S:362-369 (2001). In addition, some factors in the body are believed to aid in the repair of cartilage. For example, transforming growth factors beta (TGF-β) have the capacity to recruit progenitor cells into a chondral defect from the synovium or elsewhere when loaded in the defect. Hunziker, et al., “Repair of Partial Thickness Defects in Articular Cartilage: Cell Recruitment From the Synovial Membrane,” J Bone Joint Surg., 78-A:721-733 (1996). However, the application of growth factors to bone and cartilage implants has not resulted in the expected increase in osteoinductive or chondrogenic activity.
Each of U.S. Pat. Nos. 5,270,300 and 5,041,138, each herein incorporated by reference, describes a method for treating defects or lesions in cartilage that provides a matrix, possibly composed of collagen, with pores large enough to allow cell population and contain growth factors (TGF-β or other factors (such as angiogenesis factors)) appropriate for the type of tissue desired to be regenerated.
U.S. Patent Publication No. 2003/0044445, herein incorporated by reference, describes an osteogenic composition prepared by a process including the steps of subjecting demineralized bone to an extraction medium to produce an insoluble extraction product and a soluble extraction product, separating the insoluble extraction product and the soluble extraction product, drying the soluble extraction product to remove all or substantially all of the moisture in the soluble extraction product, and combining the dried soluble extraction product with demineralized bone particles. This process involves several steps and is quite laborious. Studies using the process have shown that the formed osteogenic composition does not have appreciably increased osteoinductive properties when compared to the demineralized bone particles to which the dried soluble extraction product is added. It was further determined that the demineralized bone from which the extraction products are extracted does not exhibit appreciably decreased osteoinductive properties when compared with its properties prior to extraction. It is thus theorized that the extraction process withdraws only a small fraction of available tissue repair factors.
Overall, current bone and cartilage graft formulations have various drawbacks. The osteoinductive factors within the matrices can be rapidly degraded and, thus, factors associated with the matrix are only available to recruit cells to the site of injury for a short time after implantation. Further, when added to a matrix, the active factors often do not appreciably increase the osteoinductive activity of the matrix or, at least, do not increase the osteoinductive activity of the matrix as much as is desirable.
Thus, it would be useful to provide increased osteoinductive activity from an osteogenic composition in more concentrated form such that increased osteoinductive activity can be seen even with little bone void space.